The present invention broadly relates to the fabrication of gear teeth on a bevel gear and, more specifically, pertains to a new and improved method, apparatus and tool for finish machining rough machined teeth of hypoid gears.
Generally speaking, a primary method of the present invention is for grinding rough-cut longitudinally curved gear teeth of a bevel gear blank by means of a tool driven in rotation about a tool axis and entailing the performance of at least one relative feed motion between the bevel gear blank and the tool.
In other words, the primary method of the present invention is for fabricating gear teeth on a bevel gear and comprises the steps of rough machining a bevel gear blank with a first axis of rotation and a first number of longitudinally curved teeth each having a longitudinally convex tooth flank and each having a longitudinally concave tooth flank while leaving a finish machining allowance on each longitudinally concave tooth flank and on each longitudinally convex tooth flank and arranging the rough machined bevel gear blank with the first axis of rotation in a hypoidally displaced relationship to a second axis of rotation of a rotary form tool.
The tool of the present invention is for grinding rough-cut longitudinally curved gear teeth of a bevel gear blank and has a conoidally helical form. The tool comprises teeth and each tooth of the teeth has a concave tooth flank and a convex tooth flank.
In other words, the tool of the present invention is a rotary form tool for finish machining tooth flanks of a first number of longitudinally curved teeth of a rough machined bevel gear blank for a hypoid gear pair.
The apparatus of the present invention is for grinding rough-cut longitudinally curved gear teeth of a helical bevel gear.
In other words, the apparatus of the present invention is for finish machining tooth flanks of a first number of longitudinally curved teeth of a rough machined bevel gear blank for a hypoid gear pair by means of a rotary form tool. The apparatus comprises a first spindle for mounting the rough machined bevel gear blank and a second spindle for mounting the rotary form tool. Means are provided for translatably adjusting at least one spindle of the first spindle and the second spindle.
A further method of the present invention is for using the inventive apparatus for finish machining tooth flanks of a first number of longitudinally curved teeth of a rough machined bevel gear blank for a hypoid gear pair by means of a rotary form tool having a second number of gear teeth and comprises the steps of mounting the rough machined bevel gear blank in a first spindle of the apparatus, employing as the rotary form tool a rotary form tool having a second number of teeth and at least one abrading surface and mounting the rotary form tool in a second spindle of the apparatus.
A still further method of the present invention is for fabricating a tool.
In other words, the still further method of the present invention is for fabricating a rotary form tool having a first axis of rotation and a first number of gear teeth for finish machining a rough machined helical bevel gear having a second axis of rotation and a second number of longitudinally curved teeth. The method comprises the steps of initially fabricating a master gear also having the first axis of rotation and corresponding to the rough machined helical bevel gear to be finish machined.
Yet a further method of the present invention is for fabricating a tool and comprises the steps of setting up data programs for associated rough-cut tools and fabricating the rough-cut tools in accordance with the set-up data programs.
In other words, the yet further method of the present invention is for fabricating a rotary form tool for finish machining a rough machined bevel gear blank. The method comprises the steps of generating a first set of dimensional data relating to the configuration of a hypoid gear master corresponding to the rough machined bevel gear blank to be finish machined.
In this specification it is to be understood that a bevel or hypoid gear pair or set or transmission comprises two bevel gears, a so-called pinion gear (usually smaller and driving) and a so-called crown or ring gear (usually larger and driven).
The manufacturing process for fabricating bevel gears and hypoid gear transmissions to be prefatorily described hereinbelow has proven its mettle both economically and qualitatively: cutting gear teeth by milling or shaping; case-hardening; and pairwise lapping. The general result is a gear pair comprising a pinion gear and a ring or crown gear which must both be marked as members of a matched pair during lapping to ensure correct installation later. The inevitable hardening distortion and its ensuing diminution of quality have always caused difficulties. Pairwise lapping reduces or eliminates individual pitch errors for improving quiet running of the gear set and for finishing the tooth flank surfaces, but radial and axial run-out errors persist. If such radial and axial run-out errors cannot be accommodated or tolerated, more suitable finish machining processes must be employed.
In the mass production of gear transmissions, especially for automobile and heavy vehicle production, cylindrical spur gears are, for instance, rough cut by milling before hardening; usually shaving or hobbing before hardening; and subsequently hardening. The quality thus attained meets the specified requirements, since hardening distortion in cylindrical gears is slight and the involute toothing employed is relatively insensitive to radial run-out errors. The arcuately-toothed bevel gears usually employed in such gear transmissions are fabricated as initially described and installed in matched pairs without detriment to the quality level of the gear transmission already established by the cylindrical gears. In order to achieve greater economy, it is desired to eliminate the fixed pairing or matching of pinion and crown gear in bevel gear sets, i.e. it is desired to eliminate hardening distortion when fine processing or finish machining bevel gear toothing or, respectively, to render distortion retrogressive after hardening.
It has long been known in practice that the toothing of bevel gears fabricated by rough cutting in the unhardened state according to the indexing method, that is bevel gears having circularly arcuate teeth, can also be ground with dished grinding wheels according to the indexing method after case hardening. This method is employed especially when utmost requirements are demanded of the gear transmission, such as in rotor drives for helicopters. In such applications, the considerably higher fabrication costs are of secondary importance.
A method for crowning longitudinally curved teeth of gears fabricated according to the continuous cutting method is known from the European Patent No. 0,022,586 and the cognate U.S. Pat. No. 4,467,567, granted Aug. 28, 1984. Both of these fine processing or finish machining methods are, however, not suited for employment in mass production, especially in automotive vehicle production, for economic reasons.
A dished or cupped grinding wheel with two conical grinding flanks for grinding helically or arcuately toothed bevel gears according to the indexing generating method is known from the German Patent No. 2,721,164. The grinding flanks confront one another and thus form a recessed inner annulus of frustro-conical cross-section.
A corresponding design of machine is described in the article "Schleifen bogenverzahnter Kegelrader in der Kleinserienfertigung" in the Technical Journal Werkstatt und Betrieb No. 118 October 1985 pages 703 to 705. The introduction to this article indicates that there had heretofore been no possibility of grinding the flanks of arcuately toothed bevel gears economically in small lots. The machine described in the article permits grinding axially displaced and non-axially displaced bevel gears with the dished or cupped grinding wheel. The process employs the indexing generating method tooth for tooth. This process is therefore especially suited for finishing bevel gears having teeth rough cut according to the indexing generating method, i.e. circularly arcuately fabricated. Under certain conditions even bevel gears having rough cut teeth with epicycloidal or involute longitudinal flank lines can be finish ground to a circular arc. Should, however, the epicycloidal or involute form deviate too greatly from a circular arc, i.e. by more than a permissible amount of grinding allowance, then this method is no longer utilizable.
A further method for fine processing or finish machining hardened helical bevel gears by generating milling is known from the wt-Zeitschrift fur industrielle Fertigung, No. 75, 1985, pages 461 to 464. In this method spiral bevel gears rough cut according to the continuous generating milling method are finish milled subsequent to case hardening. The requisite cutting tools substantially correspond to those employed for milling before hardening, but the cutting edges comprise hard metal inserts for machining after hardening. The same machine and procedure can thus be employed for machining both before and after hardening. The hard metal inserts are preferably mounted as cutting edge strips provided with a coating of polycrystalline cubic boron nitride, also known as CBN or Borazon, on their top rake or cutting faces.
An apparatus for fine processing or finish machining of gears having a shaping or shaving wheel cutter or other gear-shaped tool is known from the German Patent No. 1,161,465. In this apparatus the workpiece and the tool roll, i.e. perform generating motions, upon one another in mesh and their axes intersect. The workpiece and the tool are each connected by a respective shaft to an associated master gear. That is, the shaving wheel and the associated master gear pair as well as the workpiece gear blank and the associated master pinion pair are each fixed against rotation on one and the same shaft. This apparatus is, however, only employed for machining straight-toothed gears or helically toothed cylindrical gears. This apparatus is not utilizable for fine processing or finish machining bevel gears having longitudinally curved teeth.